The A1 is usually seen with smooth hand guards, while the A2 is usually seen with hand guards with an easy to grip surface. The carry handle in the picture doesn't appear to detach, so A3 and A4 are out. The vertical lines on the hand guard and solid carry handle lead me to guess that it is an M16A2. This is coming from a U.S. Army small arms repairer, btw. (I know it doesn't show auto or burst, before someone tries to argue that)

I read that the reply was directed at the original image, and you read it as being directed toward the comment's image. I see how that could be confusing now, and I retract my sarcastic remark, and formally apologize.

the first generation of the m161a had issues jamming because it was depoyed in a humid jungle upon its first widespread wartime use, causing the barrel/bolt/carrier assembly ti rust and inhibit free movement, the implementation of a chromed barrel and bolt partially fixed the issue. the subsequent versions that are still in use today are some of the most reliable and easily serviced firearm systems around.